TL;DR
- Free cooling uses outdoor ambient (air, water, or evaporated water) to cool the data centre directly, bypassing the mechanical chiller for some or all of the year.
- Three primary modes: direct air (outdoor air enters the room), indirect air (outdoor air cools room air via a coil or heat wheel), and waterside (cooling tower or dry cooler rejects heat from facility water).
- Drives the lowest PUE outcomes — modern UK and northern European data centres run on free cooling for 80-100 % of the year.
- Warm-water liquid cooling (DLC at W3/W4) extends the free-cooling window dramatically — if facility water can be 30-45 °C, dry coolers work year-round in nearly any climate.
Modes#
| Mode | How it works | Best climate | Risk |
|---|---|---|---|
| Direct air | Outdoor air filtered and supplied to the room | Temperate, low pollution | Humidity and contamination |
| Indirect air (heat wheel) | Two air streams exchange heat through a wheel | Wide tolerance | Wheel maintenance |
| Indirect air (coil) | Outdoor air cools inner-loop air via coil | Wide tolerance | Lower efficiency than direct |
| Dry cooler (waterside) | Outdoor air cools facility water via finned coil | Temperate, with warm-water IT | Limited in hot summer with cold water targets |
| Adiabatic-assist dry cooler | Spray water onto coil for evaporative boost on hot days | Warm temperate | Water consumption |
| Cooling tower | Open evaporative — water cooled by evaporation | Any, but water-intensive | WUE impact, Legionella risk |
Why It Matters#
Mechanical chillers are the single largest non-IT electrical load in most data centres. Eliminating chiller use for any fraction of the year drops PUE proportionally. A UK or northern European site that runs on free cooling 90 % of the year has fundamentally different economics from a hot-climate site that runs mechanical cooling year-round.
Warm-water liquid cooling is the multiplier on free cooling. If IT can be cooled by 30-45 °C facility water (ASHRAE W3/W4 classes), then dry coolers alone — no evaporation, no chiller — can reject the load in nearly any UK or EU climate year-round. This is the architecture pattern behind the lowest published PUE figures in the industry.
Free-Cooling Hours by Climate (UK Reference)#
- Northern Scotland: 99-100 % of hours achievable with dry-cooler free cooling at W4 supply.
- Central England: 95-98 % achievable; light adiabatic assist on the warmest summer days closes the gap.
- London / South-East: 90-95 % achievable; adiabatic boost more frequently needed.
- Manchester / Leeds: 96-99 % achievable; one of the most favourable UK climates for free cooling.
- Compare: Madrid / Lisbon — 60-80 % achievable; Singapore — < 10 % achievable.
Design Drivers#
- IT supply temperature target: ASHRAE A1 (18-27 °C) and A2 (10-35 °C) envelopes set the cold-air target; warm-water DLC targets are 25-45 °C inlet.
- Cooling-tower vs dry-cooler choice: tower is more efficient thermally but uses water; dry cooler uses no water but is less efficient.
- Hybrid systems: many UK facilities use dry coolers as primary and a backup chiller for the few hours per year that exceed envelope.
- Air-side economisation: direct air requires filtration to F7 or F9 at minimum; humidity control may still be needed during dry winters.
- Cooling-tower water treatment: corrosion inhibitors, biocide, blowdown management. Legionella risk needs active management (HSG274 in the UK).
Combined With Liquid Cooling#
The architecture combining warm-water DLC with dry-cooler free cooling is the current best-practice envelope for AI infrastructure in temperate climates. Specifics typically run as follows:
- Cold plates inside the server cooled to ~35-40 °C with secondary loop water.
- CDU isolates secondary from facility water; facility water can be 40-45 °C.
- Facility water rejected via dry coolers — no evaporation, no chiller — for almost the full year.
- PUE 1.10-1.15 routinely achieved; WUE near zero.
- Capex is moderate (no chiller plant); ongoing energy cost low.
Pitfalls#
- Backup chiller still required: even at 99 % free cooling, the rare extreme hours need mechanical backup. Skipping this is a single-point-of-failure on summer heatwave days.
- Air quality (direct air): salt air, pollen, urban pollution, wildfire smoke can all force return to mechanical mode.
- Humidity (direct air): below dew point causes condensation; above limits cause corrosion. ASHRAE envelopes tolerate more than older sites assume.
- Maintenance: dry coolers and cooling towers need cleaning, fan replacement, and (for towers) Legionella management.
- Climate shift: design weather data older than 5-10 years no longer reflects current peaks. Refresh weather inputs for new designs.